Which of these numbers is prime? ${16,\ 47,\ 55,\ 69,\ 77}$
A prime number has exactly two factors: 1 and itself. The factors of 16 are 1, 2, 4, 8, and 16. The factors of 47 are 1 and 47. The factors of 55 are 1, 5, 11, and 55. The factors of 69 are 1, 3, 23, and 69. The factors of 77 are 1, 7, 11, and 77. Thus, 47 is a prime number.